The present invention relates to an improved method and apparatus for producing medium- to high-carbon steel rods for use as springs and tensioning members, either twisted or untwisted, in prestressed concrete (PC). More particularly, the present invention relates to an improvement in the method of "direct heat treatment" for producing steel rods having increased tensile strength and drawability by subjecting hot-rolled steel rods to controlled cooling with a coolant.
The essence of the direct heat treatment of a medium- to high-carbon steel rod is cooling a coil of the rod substantially uniformly along the entire coil length at a suitable cooling rate so as to provide a fine pearlitic microstructure. Since the treated rod has strength and drawability properties comparable to those of a patented rod, it can be immediately drawn without patenting if the rod diameter and the specifications for the required quality so permit. However, rods used to manufacture PC tensioning members must have a large diameter and high strength, and the rod obtained by the conventional direct heat treatment has a tensile strength which is about 10 kg/mm.sup.2 lower than that of a rod that has been patented through a lead bath. Furthermore, the rods treated by the conventional direct method have a low uniformity in strength. For these reasons, patenting through a lead bath is essential in the process of manufacturing large-diameter rods for use as PC tensioning members.
Several methods have been proposed for the direct heat treatment of medium- to high-carbon steel rods, which methods have various merits and demerits as discussed below. First, the Stelmor method wherein a spiral coil expanded on a horizontal conveyor is cooled with an air blast (Japanese Patent Publication No. 15463/67) provides a rod having a reasonably uniform quality without local quenching. However, the cooling action of this method is rather weak and the resulting rod does not have a sufficient strength. The air blast does not effectively cool the overlapping portions of adjacent turns of coil, and this causes nonuniformity in the rod's strength. The second method wherein a spiral coil of rod is wound in warm water (Japanese Patent Publication No. 8536/70) or transported on a horizontal conveyor moving through warm water (Japanese Patent Publication No. 8089/71) provides a rod having a uniform quality if boiling water is used as the cooling medium. However, the product has an insuffucient tensile strength 10 kg/m.sup.2 lower than the value obtained by patenting through a lead bath, and even the tensile strength of a rod that is treated by an additional vigorous agitation with air injection (as shown in Japanese Patent Application (0PI) No. 9826/82) is 5 to 7 kg/mm.sup.2 lower than the value obtained by patenting through a lead bath. The use of subcooled boiling water (95.degree. C.) has also been proposed, and this is effective in providing increased rod strength. However, this method is not capable of producing stable film boiling, and even at elevated temperatures higher than the pearlite transformation range, nucleate boiling occurs, and the resulting local quenching yields a martensite structure, which is of course detrimental to the intended object of producing a steel rod having improved tensile strength and drawability.